Rolling ball connector

ABSTRACT

An integrated circuit assembly has pads of a chip electrically connected to pads of a substrate with rolling metal balls. A pliable material bonds the balls in movable contact with pads of the chip and substrate. Because the balls are relatively free to move, thermal expansion differences that would ordinarily cause enormous stresses in the attached joints of the prior art, simply cause rolling of the balls of the present invention, avoiding thermal stress altogether. Reliability of the connections is substantially improved as compared with C4 solder bumps, and chips can be safely directly mounted to such substrates as PC boards, despite substantial thermal mismatch.

FIELD OF THE INVENTION

[0001] This invention generally relates to electrical connectors forsemiconductor components. More particularly, it relates to a connectorbetween an integrated circuit chip and a substrate. Even moreparticularly, it relates to a connector that provides a high degree ofrelief from thermal stress to provide a very reliable joint between anintegrated circuit chip or package and a thermal expansion mismatchedsubstrate.

BACKGROUND OF THE INVENTION

[0002] Reliable interconnection of semiconductor integrated circuitchips and supporting substrates depends on avoiding stresses, includingthermal expansion stresses, that can crack interconnects. Usuallyintegrated circuits are mounted on supporting substrates made ofmaterial with a coefficient of thermal expansion that differs from thecoefficient of thermal expansion of the material of the integratedcircuit. For example, the integrated circuit may be formed ofmonocrystalline silicon with a coefficient of thermal expansion of2.5×10⁻⁶ per ° C. and the supporting substrate may be formed of aceramic material, such as alumina, with a coefficient of thermalexpansion of 5.8×10⁻⁶ per ° C. In operation, the integrated circuit chipgenerates heat which raises the temperature of both the chip and thesupporting substrate. Because of different temperatures and differentcoefficients of thermal expansion, the chip and substrate expand andcontract different amounts. This difference in expansion imposesstresses on connections, such as the relatively rigid C4 solder bumpsthat are frequently used to provide an area array interconnectionbetween a chip and a substrate. The stress on the solder bumps isdirectly proportional to (1) the magnitude of the temperaturedifference, (2) the distance of an individual bump from the neutral orcentral point of the solder bump array, and (3) the difference in thecoefficients of thermal expansion of the material of the semiconductordevice and the substrate, and inversely proportional to the height ofthe solder bond, that is the spacing between the IC chip and the supportsubstrate.

[0003] Several factors are currently compounding the problem. As thesolder bumps become smaller in diameter in order to accommodate the needfor a greater density of interconnects between chip and substrate, theoverall height of each solder bump decreases, reducing the fatigue lifeof the solder bumps. In addition, integrated circuit chip sizes areincreasing which increases the distance of the outer solder bumps fromthe neutral point of the solder bump array, which in turn reduces thefatigue life of the solder bump. Furthermore, chips are now beingdirectly mounted on substrates, such as PC boards, that havesubstantially larger coefficients of thermal expansion than ceramic,adding substantially to the stress on connectors. Thus, a bettersolution is needed that provides a way to reduce thermal stress and toprovide a more reliable electrical connection, and this solution isprovided by the following invention.

SUMMARY OF THE INVENTION

[0004] It is therefore an object of the present invention to provide asemiconductor assembly having connections with improved reliability.

[0005] It is another object of the present invention to provide asemiconductor assembly having connections that reduce thermal expansionmismatch stress and resist thermal fatigue and cracking due to thermalcycling.

[0006] It is another object of the present invention to provide anelectrical connection between two substrates wherein a solid conductorin the connection is in substantially movable electrical contact with atleast one of the substrates.

[0007] It is another object of the present invention to provide apliable conductive material to facilitate the substantially movableconductor.

[0008] It is another object of the present invention to provide amovable conductor that can roll or slide.

[0009] It is another object of the present invention to provide amovable conductor that can stretch or bend a magnitude exceeding theelastic limit of a uniform metal.

[0010] It is a feature of one embodiment of the present invention thatthe movable contact is substantially elastic movement.

[0011] It is a feature of the present invention that the conductivematerial is an adhesive that bonds and provides electrical connectionwhile permitting elastic movement of the movable conductor.

[0012] It is an advantage of the present invention that thermal stressis avoided despite large disparities in thermal expansion coefficientbetween chip and substrate.

[0013] It is an advantage of the present invention that a semiconductorassembly has good electrical connections while thermal stress is avoideddespite large disparities in thermal expansion coefficient between chipand substrate.

[0014] These and other objects, features, and advantages of theinvention are accomplished by providing a semiconductor assemblycomprising a first substrate and a second substrate. The first substratehas a first contact pad and the second substrate has a second contactpad. In addition, the assembly includes a solid conductor and a materialbonding the solid conductor wherein the solid conductor is insubstantially movable contact with the first contact pad.

[0015] In one embodiment an integrated circuit chip has padselectrically connected to pads of a substrate through a metal ball and apliable material. The pliable material bonds the metal ball insubstantially movable contact with the pads. Because the ball isrelatively free to move through the pliable material, thermal expansiondifferences that would ordinarily cause stress in an immovable jointsimply cause the ball to roll across the pads as the chip and substratefreely expand or contract at their different rates. Thus, stress isavoided and reliability of the connection is substantially improved ascompared with fixed connectors of the prior art.

BRIEF DESCRIPTION OF THE DRAWINGS

[0016] The foregoing and other objects, features, and advantages of theinvention will be apparent from the following detailed description ofthe invention, as illustrated in the accompanying drawings, in which:

[0017]FIG. 1 is a cross sectional view showing a chip and substrateelectrically connected by ball conductors and conductive paste;

[0018]FIG. 2 is a cross sectional view of a ball conductor comprising acore and a highly conductive coating, the core comprising a base metalor a polymer, the highly conductive coating comprising a noble metal,such as gold;

[0019]FIG. 3a is a cross sectional views of an alternate embodiment inwhich the ball conductor has conductive paste on one side and a stablefillet on the other side, the stable fillet being solder or a curedconductive adhesive;

[0020]FIG. 3b is a cross sectional views of another embodiment in whichthe ball conductor has stable fillets on both sides, the stable filletbeing solder or a cured conductive adhesive;

[0021]FIG. 3c is are a cross sectional view of the embodiment of FIG. 3bin a stretched configuration in response to differential thermalexpansion of chip and substrate;

[0022]FIGS. 4a-4 c are cross sectional views illustrating the steps offabricating the connection of FIG. 1;

[0023]FIG. 5a is a cross sectional view and FIGS. 5b-5 c threedimensional views showing the application of the present invention forchip or wafer burn-in.

[0024]FIG. 6a is a cross sectional view showing an embodiment of theinvention in which a frame adds mechanical strength;

[0025]FIG. 6b is a cross sectional view showing an embodiment of theinvention in which stable fillets, such as C4 solder bumps or curedadhesive, add mechanical strength;

[0026]FIG. 7 is a three dimensional view showing chips mounted on cardsaccording to the present invention;

[0027]FIG. 8 is a cross sectional view showing an alternate embodimentof the invention in which conductive adhesive is formed as columns, theball conductor being embedded in each column; and

[0028]FIG. 9a and 9 b are cross sectional views showing an alternateembodiment of the invention in which a plurality of ball conductorscomprising a core and a highly conductive coating are deformed duringmounting to ensure connection to all ball conductors.

DETAILED DESCRIPTION OF THE INVENTION

[0029] In one embodiment, the present invention provides an integratedcircuit assembly in which pads of a first substrate are electricallyconnected to pads of a second substrate with rolling metal balls. Apliable material bonds the balls in movable contact with the pads.Because the balls are relatively free to move, thermal expansiondifferences that would ordinarily cause enormous stresses in theattached joints of the prior art, simply cause rolling of the balls ofthe present invention, avoiding stress altogether. Thus, reliability ofthe connections is substantially improved, and silicon chips can besafely directly mounted to such substrates as PC boards, despitesubstantial thermal expansion mismatch.

[0030] Conductive ball 20 and conductive paste 22 are used to connecteach pad 24 of chip 26 with pad 27 of substrate 28, as shown in FIG. 1.Conductive ball 20 preferably has a dimension approximately equal to thewidth of pad 24, the connection between pad 24 and pad 27, or theseparation distance between pad 24 and pad 27, as shown in FIG. 1.Conductive ball 20 can be a solid metal ball, formed of a material suchas gold or copper. Conductive ball 20′ can also have a layered structurewith base 30 and highly conductive coating 32 as shown in FIG. 2. Base30 is formed of a base metal, such as copper, aluminum, or iron, or itcan be formed of a polymer with significantly greater compliance than ametal or solder, such as silicone filled with particles of alumina orsilica. The particles improve the mechanical properties of the ball,making it more stiff. Alternatively, base 30 can be formed of a metalfilled polymer, such as silicone filled with gold, silver, or graphite.Although coating 32 is preferred, in this case it may not be necessaryto provide a highly conductive coating. Highly conductive coating 32 ispreferably a noble metal, such as gold, to avoid corrosion and providelow electrical resistance electrical contact, as is well known in theart. Conductive coating 32 can also be formed of copper,copper-nickel-gold, or another metal. Conductive coating 32 is providedwith a thickness in the range from 200A to 100 micrometers, morepreferably in the range 0.5 to 5 micrometers.

[0031] Conductive paste 22 is formed of a material such as a metalfilled conductive paste, for example, Polymer Metal Composite Paste(PMC) and Epo-tek, manufactured by Epoxy Technology. PMC is formed of ahigh temperature epoxy filled with a metal, such as gold. PMC has anresistivity of 10-30 micro-ohm-cm. and a viscosity of 75,000 to 200,000P_(S).

[0032] Conductive paste 22 makes electrical contact with pad 24, pad 27,and with conductive ball 20. While continuing to maintain electricalcontact with paste 22, conductive ball 20 can roll or otherwise movethrough conductive paste 22. Preferably, paste 22 wets to ball 20 andmaintains this wetting contact to ball 20 as ball 20 moves. Thus,electrical connection between pads 24 and 26 is maintained through paste22 and ball 20 while chip 26 moves laterally with respect to substrate28 in response to differential thermal expansion forces. Ball 20 rollsas chip 26 and substrate 28 move. In addition to providing improvedreliability with respect to thermal expansion, this embodiment of theinvention also provides advantage in that chips are mounted face downwithout the use of lead containing solder, avoiding a source of alphaparticles that can cause soft errors while chip 26 is operating.

[0033] While providing paste 22 on both sides of ball 20 allows freemovement for both chip 26 and substrate 28 as ball 20 moves throughpaste 22, substantial movement and stress-free results can also beachieved by providing conductive paste 22 on only one side of ball 20.Stable fillet 38, such as solder or cured conductive adhesive is used onthe other side, as shown in FIG. 3a. Conductive adhesives include curedPMC and cured Epo-tek. In the case illustrated in FIG. 3a, conductiveball 20 is fixed to chip 26 with stable fillet 38 while conductive pasteis provided between ball 20 and substrate 28. Of course, the roles ofchip 26 and substrate 28 can be reversed, and stable fillet 38 can beapplied on the side of substrate 28 while paste 22 is applied on theside of chip 26.

[0034] Furthermore, if ball 20, 20′ is sufficiently compliant for thesize of chip 26, temperature range, and height, stable fillets 38 can beprovided on both sides, as shown in FIG. 3b. The ability of ball 20′ tostretch accommodates differential thermal expansion, as shown in FIG.3c. Gold coated polymer ball 20′ provides enormous compliance comparedwith standard metal ball 20 or standard lead-tin solder bumps.Alternatively, gold coated polymer ball 20′ can be replaced with a goldcoated polymer column to further improve reliability. Thus, in manyapplications, rolling or sliding of ball 20 can be replaced withstretching or bending of gold coated polymer ball 20′, as shown in FIG.3c. Because of the enormously greater stretching permitted by polymerball 20′ as compared with metal ball 20 or a C4 solder bump, a connectorformed with polymer ball 20′ is considered to provide a substantiallymovable contact there between. Ball 20′ can now be attached on bothsides with stable fillets 38, such as solder or a cured adhesive whilestill providing a substantially low-stress, flexible contact. Because ofthe enormous elastic flexibility of polymer ball 20′, chip 26 andsubstrate 28 still have substantial freedom to move, the movementstretching gold coated polymer ball 20′ without exceeding its elasticlimit. Stable fillets 38 on either side of polymer ball 20′ haveadvantage in that they provide a strong permanent mechanical connectionof chip 26 with substrate 28.

[0035] Steps in fabrication of the assembly are illustrated in FIGS.4a-4 c. In the first step, conductive paste deposit 40 a, is applied toeach pad 24 of chip 26, as shown in FIG. 4a. Similarly, a conductivepaste deposit is applied to each pad of substrate 28 (not shown). In thenext step, vacuum holder 42 is used to pick up array 44 of balls 20corresponding to the location of pads 24 and 27 on chip 26 and substrate28. Array 44 of balls 20 is pushed down into paste deposits 40 a, asshown in FIG. 4c, on chip 26 and vacuum is released, providing balls 20in electrical connection with compressed paste 22, which in turn iselectrically connected to pad 24. In the next step, substrate 28, alsohaving paste deposits, is aligned and pressed down on balls 20, as shownin FIG. 1. The mechanical and electrical connection between chip 26 andsubstrate 28 through ball 20 and paste 22 on either side of ball 20 isnow complete. Alternatively, array 44 of balls 20 can first be mountedon substrate 28, and chip 24 mounted thereon.

[0036] A fabrication technique similar to that illustrated in FIGS. 4a-4c can be used to provide a stable fillet on one or both sides of solderball 20. Stable fillets include conductive polymeric adhesive,conductive epoxy, and solder. In this case, a curing step or elevatedtemperature reflow step is provided after the ball has been inserted inthe deposit of conductive material shown in FIG. 4c. For the embodimentof the invention having cured epoxy on both sides of ball 20, a singlecuring step can be used to cure the epoxy on both sides. Solder pastecan be used to provide solder fillets on one or both sides of ball 20,and in this case, an elevated temperature reflow is used in place of acure.

[0037] The present invention is applicable for temporary chip attach fortest and burn-in, as shown in FIGS. 5a-5 d. Individual chips 26 aretemporarily mounted on test and burn-in substrates 28′ with the ball 20and conductive paste 22 connectors of the present invention, as shown inFIGS. 5a and 5 b. Similarly, entire wafer 46 is mounted on a temporarytest head 48 with the ball 20 and conductive paste 22 connectors of thepresent invention, as shown in FIGS. 5c and 5 d. Expansion differencesas temperature changes for elevated temperature test or burn-in areaccommodated by the movable contacts. After testing and burning-in ofchips 26 or wafer 46 is complete, chips 26 or wafer 46 can be removedfrom test and burn-in substrates 28′ or test head 48. Conductive paste22 permits disassembly after test and burn-in are complete without anydamage to chip 26 and substrate 28 or to wafer 46 and test head 48.

[0038] While adhesive properties of paste 22 are sufficient to hold chip26 in place on substrate 28, mechanical hold-down 50 can also be used,as shown in FIG. 6a to prevent excessive movement of chip 26. Mechanicalhold-down 50 is connected to substrate 28 with adhesive or solder layer52. Space 54 is provided, permitting the small amount of lateralmovement of chip 26 needed to accommodate thermal expansion whilemechanical hold-down 50 prevents vertical movement. Mechanical hold-down50 can also serve to electrically shield chip 26 and to provide a heatsink for chip 26. In an alternative embodiment, substantially fixedcontacts, such as C4 solder bumps or cured conductive adhesive 56, isused near neutral point 0.000 (or center) of chip 26, providing certainmechanical connection, while rolling ball connectors 20, 20′ are usedtoward the perimeter of chip 26 to accommodate differential thermalexpansion between chip and substrate, as shown in FIG. 6b.

[0039] Multiple chips connected with movable connectors of the presentinvention can be provided on a single substrate, as shown in FIG. 7.Substrate 28 can be a card, flex, ceramic substrate, lead frame, or anyother substrate.

[0040] In addition to stable fillets, conductive adhesive can be formedas columns, 60 as shown in FIG. 8, to provide a higher standoff and morereliable joint. Conductive ball 20 serves to stiffen column 60permitting column 60 to taller. Column 60 is formed of electricallyconductive adhesive (ECA), such as polymer metal composite paste, anepoxy filled with metal particles, as described herein above. The ECA isfirst deposited on substrate 28. Then an array of balls is placed with atechnique such as vacuum suction cups. ECA is also deposited on matchingpads for chip 26 and the chip is aligned and placed on substrate 28.

[0041] In an alternate embodiment, polymer ball 20′ of FIG. 2 isprovided as shown in FIG. 9a, and then compressed and deformed duringmounting, as shown in FIG. 9b, ensuring a good connection as heightmismatch among pads is accommodated. Pads 24, 27 are formed ofmultilevel thin films comprising copper and nickel. The pad surfaces arefirst roughened by plating palladium dendrites on the nickel pads, as iswell known in the art. After ball 20′ is placed between chip pad 24 andsubstrate pad 27, mechanical force is provided against the roughened padsurfaces to create a reliable interconnection. This technique can beused for a reliable chip-substrate connection or a substrate-printedwiring board connection. Alternatively solder can be used to provideconnection between deformable polymer ball 20′ and pads 24, 27. Polymerball 20′ can also be configured in a shape, such as a column, toincrease height and further improve reliability.

[0042] While several embodiments of the invention, together withmodifications thereof, have been described in detail herein andillustrated in the accompanying drawings, it will be evident thatvarious further modifications are possible without departing from thescope of the invention. Nothing in the above specification is intendedto limit the invention more narrowly than the appended claims. Theexamples given are intended only to be illustrative rather thanexclusive.

[0043] What is claimed is:

1. A semiconductor assembly comprising: a first substrate, a secondsubstrate, and a contact there between, said first substrate having afirst contact pad, said second substrate having a second contact pad,said contact comprising: a solid conductor, said solid conductor havinga dimension about equal to a dimension of said contact; and a materialbonding said solid conductor to said first contact pad wherein saidsolid conductor is in substantially movable electrical contact with saidfirst contact pad.
 2. A semiconductor assembly as recited in claim 1 ,wherein said first substrate comprises an integrated circuit chip, achip carrier, a printed circuit board, a flex, or a card.
 3. Asemiconductor assembly as recited in claim 1 , wherein said secondsubstrate comprises a an integrated circuit chip, a chip carrier, aprinted circuit board, a flex, or a card.
 4. A semiconductor assembly asrecited in claim 1 , wherein said solid conductor comprises a metal. 5.A semiconductor assembly as recited in claim 4 , wherein said solidconductor comprises a metal ball.
 6. A semiconductor assembly as recitedin claim 1 , wherein said solid conductor comprises a polymer.
 7. Asemiconductor assembly as recited in claim 6 , wherein said polymercomprises a conductor coated polymer.
 8. A semiconductor assembly asrecited in claim 7 , wherein said conductor coated polymer comprises aball.
 9. A semiconductor assembly as recited in claim 6 , wherein saidpolymer comprises a conductive polymer.
 10. A semiconductor assembly asrecited in claim 9 , wherein said conductive polymer comprises a metalfilled polymer.
 11. A semiconductor assembly as recited in claim 6 ,wherein said polymer provides a resilient connector.
 12. A semiconductorassembly as recited in claim 1 , wherein said bonding material comprisesa pliable material.
 13. A semiconductor assembly as recited in claim 12, wherein said pliable material comprises a metal filled epoxy.
 14. Asemiconductor assembly as recited in claim 1 , wherein said bondingmaterial provides a stable fillet.
 15. A semiconductor assembly asrecited in claim 14 , wherein said stable fillet comprises an adhesive.16. A semiconductor assembly as recited in claim 15 , wherein saidadhesive comprises a conductive adhesive.
 17. A semiconductor assemblyas recited in claim 14 , wherein said stable fillet comprises a solder.18. A semiconductor assembly as recited in claim 1 , wherein saidsubstantially movable contact involves rolling.
 19. A semiconductorassembly as recited in claim 1 , wherein said substantially movablecontact comprises sliding.
 20. A semiconductor assembly as recited inclaim 1 , wherein said substantially movable contact comprisesstretching or bending.
 21. A semiconductor assembly as recited in claim20 , wherein said stretching or bending is of a magnitude exceeding theelastic limit of lead-tin solder.
 22. A semiconductor assembly asrecited in claim 1 , further comprising a physical stop to preventexcess chip movement of said first substrate.
 23. A semiconductorassembly as recited in claim 1 , wherein said first contact pad is on asurface, said surface defining a plane, said solid conductor being inmovable contact in a direction parallel to said plane.
 24. Asemiconductor assembly as recited in claim 23 , wherein said solidconductor is capable of rolling in a direction parallel to said plane.25. A semiconductor assembly as recited in claim 23 , wherein saidmovable contact comprises substantial elastic deformation.
 26. Asemiconductor assembly as recited in claim 1 , wherein saidsubstantially movable contact comprises stretching or bending.
 27. Asemiconductor assembly as recited in claim 1 , further comprising aplurality of said movable electrical contacts.
 28. A semiconductorassembly as recited in claim 27 , further comprising at least onesubstantially fixed contact.
 29. A semiconductor assembly as recited inclaim 28 , wherein said at least one substantially fixed contact islocated near a neutral point of said first substrate.
 30. Asemiconductor assembly as recited in claim 1 , further comprising asecond material bonding said solid conductor to said second contact padwherein said solid conductor is in substantially movable electricalcontact with said second contact pad.
 31. A semiconductor assembly asrecited in claim 1 , further comprising a second material bonding saidsolid conductor to said second contact pad wherein said solid conductoris in substantially fixed electrical contact with said second contactpad.
 32. A semiconductor assembly as recited in claim 1 , said contacthaving a height, said dimension being about equal to said height.
 33. Asemiconductor assembly as recited in claim 1 , said first pad having awidth, said dimension being about equal to said width.
 34. A method offabricating an assembly, comprising the steps of: (a) providing a firstsubstrate and a second substrate, said first substrate having a firstcontact pad, said second substrate having a second contact pad; (b)providing a first material on said first contact pad; (c) providing asolid conductor on said first material, said solid conductor having adimension about equal to a dimension of the contact; (d) providing asecond material on said second substrate; and (e) bonding said secondsubstrate to said solid conductor, wherein said solid conductor is insubstantially movable electrical contact with said first contact padafter fabrication is complete.
 35. A method of fabricating as recited inclaim 34 , wherein said first substrate comprises a burn-in substrateand said second substrate comprises a chip and wherein said methodfurther comprises the step of burning in said chip.
 36. A method offabricating as recited in claim 35 , further comprising the step ofremoving said chip from said substrate and mounting said burned-in chipin a final package.
 37. A method of fabricating as recited in claim 34 ,wherein said first substrate comprises a temporary test head and saidsecond substrate comprises a wafer and wherein said method furthercomprises the step of burning-in said wafer.
 38. A method of fabricatingas recited in claim 37 , further comprising the step of removing saidwafer from said test head, dicing said burned-in wafer into chips, andmounting said burned-in chip into final packages.
 39. A method offabricating as recited in claim 34 , wherein said first and secondsubstrates each have an array of contact pads and said providing step(c) comprises providing an array of solid conductors simultaneously, onefor each contact pad.
 40. A method of fabricating as recited in claim 39, wherein said providing step (c) comprises providing each saidconductor of said array of solid conductors with a vacuum device havingan array of vacuum ports.